Deposit return query - please help!

Campaigning and Policy

This is the situation.

The tenants repeatedly caused water damage to a flat below them by mis-using the bathroom, but (largely) deny any responsibility.

I spent several hours investigating the problem and found that they were undoubtedly the case of the damage. (The flat has a history of water leakage problems, unrelated to this event and mostly pre-dating my ownership).

I repaired the damage to the flat below myself.

I formally wrote to the tenants explaining that they were the cause of the problem and for any further damage, they would be held responsible.

Although the problem mostly stopped,(apart from some minor damage much later, which I also repaired), the tenants were upset, and created a great deal of bad feeling between other people living in the house and myself.

To Conclude.
The tenants have taken up a lot of my time, energy and grief, by my having to:
...investigate the causes of repeated damage,
...deal with upset neighbours and co-freeholders
...repair damage and decorations

How do I calculate how much to fairly deduct from the deposit?

Thanks for reading this. Any advice very welcome.

Cheers,
Colin